1. Field of the Invention
The present invention relates to amplifier circuits which employ field effect transistors (FETs).
2. Description of the Related Art
FIG. 1 is a block diagram of a part of a conventional portable telephone set. The portable telephone set includes an input/output circuit part 1, a baseband circuit part 2, a high-frequency circuit part 3, an antenna 4 and a power supply part 5.
The input/output circuit part 1 includes a keyboard, a display part, a microphone and a speaker. The baseband circuit part 2 converts an analog voice signal supplied from the microphone of the input/output circuit part 1 into a digital signal. Then, the baseband circuit part 2 performs voice coding, channel coding, scrambling, and digital modulating processes. Finally, the baseband circuit part 2 converts the processed digital signal into an analog signal, which is supplied to the high-frequency circuit part 3. Further, the baseband circuit 2 converts an analog signal supplied from the high-frequency circuit part 3 into a digital signal. Then, the baseband circuit 2 performs digital demodulating, descrambling, channel decoding and voice decoding operations. Then, the baseband circuit part 2 supplies the resultant analog voice signal to the speaker of the input/output circuit part 1.
The high-frequency circuit part 3 modulates the analog signal from the baseband circuit part 2 and performs a power amplifying operation on the modulated signal. Also, the high-frequency circuit part 3 amplifies a signal received via the antenna 4 and demodulates the amplified signal. The power supply part 5 supplies electricity to the input/output circuit part 1, the baseband circuit part 2 and the high-frequency circuit part 3.
FIG. 2 is a circuit diagram of the configuration of the high-frequency circuit part 3. The high-frequency circuit part 3 includes a transmit circuit part 7, a receive circuit part 8 and a switch circuit 9. The switch circuit 9 connects an output terminal 7A of the transmit circuit part 7 to the antenna 4 at the time of transmitting a signal, and connects an input terminal 8A of the receive circuit part 8 to the antenna at the time of receiving a signal.
The transmit circuit part 7 includes a modulator 10, a voltage-controlled oscillator (VCO) 11, a multiplier 12, an amplifier 13, a surface acoustic wave (SAW) filter 14, and a power amplifier 15. The modulator 10 modulates the signal supplied from the baseband circuit part 2. The VCO 11 generates a signal necessary to perform an up conversion of the signal output by the modulator 10. The multiplier 12 multiplies the signal from the modulator 10 by the signal from the VCO 11 so that the modulated signal is changed to a high-frequency signal. The amplifier 13 amplifies the high-frequency signal output from the multiplier 12. The SAW filter 14 functions as a band-pass filter and filters the amplified high-frequency signal. The power amplifier 15 performs a power amplifying operation on the amplified high-frequency signal.
The receive circuit part 8 includes an amplifier 16, a SAW filter 17, a VCO 18, an amplifier 19, a multiplier 20 and a demodulator 21. The amplifier 16 amplifies the signal received via the antenna 4. The SAW filter 17 functions as a bandpass filter and filters the amplified received signal. The VCO 18 generates a signal necessary to perform a down conversion on the signal from the SAW filter 17. The amplifier 19 amplifies the signal output by the VCO 18. The multiplier multiplies the signal from the SAW filter 17 by the amplified signal from the amplifier 19 so that the baseband signal of the received signal can be retrieved. The demodulator 10 demodulates the output signal of the multiplier 20 and reproduces the original signal.
FIG. 3 is a circuit diagram of the power amplifier 15. The power amplifier 15 is formed in a monolithic microwave integrated circuit (MMIC) formation, and is comprised of two amplifier circuits 24 and 25, an input terminal 23 and an output terminal 26. An input signal IN is applied to the input terminal 23, and an output signal OUT is output via the output terminal 26.
The amplifier circuit 24 includes a depletion type Shottky gate field effect transistor (so called D-type MESFET) 27, which functions as an amplifying element, and a capacitance element (capacitor) 28 preventing a DC component from being applied to the amplifier circuit 25. The amplifier circuit 24 includes a drain voltage input terminal 29 supplied with a positive drain voltage VDD1 from a drain voltage source. The voltage VDD1 is equal to, for example, +4 V. The amplifier circuit 24 includes a gate bias circuit 30, which supplies the gate of the D-type MESFET 27 with a negative gate bias voltage VGB1 equal to, for example, -1.5 V. The amplifier circuit 24 includes a gate bias source voltage input terminal 31 supplied with a negative gate bias source voltage VGG1 from a gate bias source. The voltage VGG1 is equal to, for example, -4.0 V. The amplifier circuit 24 includes resistors 32 and 33, which divide the gate bias source voltage VGG1 to thereby generate a gate bias voltage VGB1.
The amplifier circuit 25 includes a D-type MESFET 34, which functions as an amplifying element, and a capacitance element (capacitor) 35 preventing a DC component from being applied to the output terminal 26. The amplifier circuit 25 includes a drain voltage input terminal 36 supplied with a positive drain voltage VDD2 supplied from a drain voltage source. The voltage VDD2 is equal to, for example, +5.8 V. The amplifier circuit 25 includes a gate bias circuit 37, which supplies the gate of the D-type MESFET 34 with a negative gate bias source voltage VGG2 equal to, for example, -4.0 V. The amplifier circuit 25 includes resistors 39 and 40, which divide the gate bias source voltage VGG2 to thereby generate a gate bias voltage VGB2.
The portable telephone set as described above employs the power amplifier 15 of the transmit circuit part 7 of the high frequency circuit part 3, the power amplifier 15 using the D-type MESFETs 27 and 34. This structure requires the positive power sources respectively functioning as the drain voltage sources for the D-type MESFETs 27 and 34 and the negative power sources respectively functioning as the gate bias sources for the D-type MESFETs 27 and 34. In practice, a DC-DC converter is used to realize the positive and negative power sources. This increases the production cost and prevents down sizing.